Polyethersulfone (PES) membranes with nano-porous surface were fabricated by addition of potassium perchlorate (KClO4) as an additive in the casting solution. The membranes were prepared from PES/polyvinylpyrrolidone (PVP)/KClO4/dimethylacetamide (DMAc) via phase inversion induced by immersion precipitation technique. Mixture of water and isopropyl alcohol (IPA) was used as coagulation medium. The viscosity of PES casting solution increased with addition of KClO4. The contact angle measurements demonstrated that the hydrophilicities of membrane enhanced by increasing the KClO4 concentration in the casting solution. According to SEM analysis, it was found that the addition of KClO4 had a significant influence on structure of the membrane skin layer and the sub-layer. Membranes with dense surface and sub-layer were formed at this condition. The surface AFM images indicated that the addition of KClO4 in the casting solution resulted in nano-structure membranes with surface small pore size. The effect of KClO4 on the membrane performance was investigated using cross-flow filtration of pure water and non-skim milk as feeds. The pure water flux through the prepared membranes declined by adding 1, 3 and 5 wt.% of KClO4 in the casting solution. On the other hand, milk water permeation and protein rejection of prepared membranes were improved. (C) 2010 Elsevier B.V. All rights reserved.